The present invention relates to transmission of frames on a radio communication channel, wherein the frames contain bursts modulated with different modulation schemes.
In radio communication networks, several modulation schemes can be used at the same transmitter depending on the receiver to which the data are destined. The modulation scheme may be, for example, determined for each receiver so that a predefined carrier to interference ratio is ensured. Usually, it is advantageous to select modulation schemes with different modulation efficiency, the modulation efficiency being defined as the number of data bit per modulation symbols. The higher the number of bit per modulation symbols, the more efficient the modulation. Indeed, a modulation with a high efficiency will be used to communicate with receivers located in a zone subjected to a low amount of interference while a more resistant modulation (i.e. with a lower efficiency) will be used to communicate with receivers located in a zone with a high level of interference.
FIG. 1 represents a simplified radio communication network where several modulations schemes M1, M2, M3 can be used for communicating between a base station 10 and several end-user radio terminals 11, 12, 13. Depending on the location of the radio terminals 11, 12, 13 or on the end-user profile, the core network associates an appropriate modulation scheme M1, M2, M3 with a corresponding modulation efficiency to each radio terminal respectively 11, 12, 13. As a consequence, base station 10 using TDM (time division multiplex) for communicating with radio terminal 11, 12, 13 sends frames containing bursts B1, B2, B3 . . . modulated respectively with modulations schemes M1, M2, M3 and destined respectively to radio terminals 11, 12, 13 . . .
Such mechanisms increase substantially the capacity of the network and are especially used in fixed wireless networks such as Local Multipoint Distribution Services (LMDS) networks. In fixed wireless networks, the end-users are not moving, the interference level affecting each user is thus easy to determine, and a static association of a modulation scheme to each user is easily possible. However, in mobile communication networks, such a mechanism is also possible provided the interference level experienced at each end-user radio terminal is reported to the core network at regular time intervals so that the modulation scheme is accordingly dynamically modified.
The data modulated with different modulation schemes (bursts) destined to different users are then encapsulated in a frame at the transmitter and transmitted on the air interface.
Usual modulation schemes can be represented by a modulation constellation. The term modulation constellation refers to the geometric representation of the modulation symbols constituting the modulation. Each point on the constellation represents the coordinates of a modulation symbol. Represented in a polar coordinate system, the constellation gives for each modulation symbol its phase and its amplitude. A characteristic value of a modulation is its average transmitted power, which can be calculated as the sum of the power (proportional to the square of amplitude) of each modulation symbol divided by the total number of modulation symbols. For example, the average transmitted power for a QPSK modulation equals 2*a2 if the modulation constellation comprises four modulations symbols belonging to the groups of (P;Q):=(+/−a;+/−a) (a being the amplitude of the signal), for a 16 QAM 10*a2 if the modulation constellation comprises sixteen modulations symbols belonging to the groups of (P;Q)=(+/−a;+/−a) (+/−3a;+/−a) (+/−a;+/−3a) (+/−3a;+/−3a) and for a 64 QAM 40*a2.
In a frame comprising bursts modulated with different modulation schemes, the average transmitted power observed at the scale of a frame varies (e.g. jumping from 2*a2 if a burst is modulated with a QPSK modulation to 40*a2 if the next burst is modulated with a 64 QAM and so on).
Varying average transmitted power has the disadvantage to cause unpredictable interference in the network that cannot be easily dealt with while trying to increase network capacity. Indeed, the average transmitted power variations may increase inter-symbol -interference.
A known solution is shown on FIG. 2 and consists in putting a multiplier stage 23 at the output of the digital part of the modulator 20 (i.e. after a mapping module 21 and a digital transmission filter 22) in the transmission chain to regulate the transmitted power and keep it constant. This multiplier stage 23 has the function of an automatic gain controller which selectively amplifies the bursts modulated with a modulation having a low average transmitted power while attenuating the bursts having a high average transmitted power to a predefined constant power value P.
The multiplier stage 23 must be controlled by the mapping module 21 in order to know which modulation has been applied to the current burst and as a consequence which gain it should apply to this burst. Then, the digital signal is supplied to a digital/analog converter 24 and transmitted on the radio communication channel.
This solution presents the disadvantage to require a complex control mechanism to synchronise the mapping module 21 with the multiplier stage 23. Indeed a delay must be taken into account to control the multiplier stage 23 what results in an increased complexity of the transmit chain by requiring additional ASIC gates in the transmitter.
Moreover, the inter-symbol-interference is not eliminated at the transition between two different modulation schemes and some symbols are lost between the digital transmission filter 22 and the multiplier stage 23. In order to improve this method, a preamble or guard symbols must be included between two bursts modulated with different modulations. This reduces the bad effects of the inter-symbol interference since the lost symbols are dummy symbols belonging to the preamble and not traffic data. However, such a solution reduces the capacity of the network since a part of the available bandwidth is used for the transmission of dummy symbols.
Moreover, this solution implies a double rounding operation: one rounding at digital filter 22 and one rounding at multiplier 23, which improves the quantization noise and degrades performances.
A particular object of the present invention is to provide a method for transmitting frames containing bursts modulated with modulation schemes having different modulation efficiency which generates a low inter-symbol interference and increases the capacity of the network.
Another object of the invention is to provide a transmitter for generating and transmitting frames according to the above mentioned method.